SULFIDE MINERALIZATION IN THE OLARY-BLOCK, SOUTH AUSTRALIA - EVIDENCEFOR SYNTECTONIC TO LATE-STAGE MOBILIZATION

The South Australian portion of the Willyama Inliers hosts a diversity of small sulphide and uranium deposits and numerous outcropping gossa ns. This fact, together with geological similarities to the adjacent B roken Hill Block has led to extensive exploration. A broad classificat ion distinguishes two main types of sulphide mineralisation: 1) strati form iron sulphide-dominated (+/-Cu, Zn, Co) deposits which occur wide spread within specific stratigraphic intervals, and stratabound occurr ences of syn-depositional to diagenetic origin which show some structu ral control; 2) syn-tectonic to post-peak metamorphic replacement and vein-type deposits (Fe-Cu-Au and Cu-Zn-Pb), which are hosted by fractu res and within faults and shear zones. These occurrences show no strat igraphic control and are not spatially related to type 1 mineralisatio n. Late-stage deposits also differ from stratiform/stratabound mineral isation in their texture, mineral assemblage and geochemical compositi on. Much of the sulphide mineralisation in the Olary Block has been in terpreted as resulting from rift-associated syn- to diagenetic process es, such as hot spring exhalations and base metal precipitation along reduction-oxidation interfaces. Subsequent granitic intrusive, high gr ade metamorphic and multiphase deformation events would have induced r emobilisation and redeposition of sulphides in a variety of epigenetic modes. However, a detailed petrographic and geochemical study of sulp hide mineralisation in the Olary Block demonstrates that due to the la ck of abundant pervasive fluids, translocation and modification of pre existing sulphides were restricted to less than a few centimetres. Ins tead, widespread syn-tectonic to epigenetic (i.e., post-peak metamorph ic) mobilisation of ore constituents occurred to form retrograde sulph ide mineralisation as well as multiple generations of late-stage vein deposits. These epigenetic deposits are genetically unrelated to syn-s edimentary and diagenetic occurrences, an aspect of significance for e xploration in the Olary Block. Temporal separation of peak metamorphis m in deeper crustal levels from its occurrence in shallow levels, peri odic tectonic disturbances and repeated seismic pumping are processes believed to have resulted in intermittent mobilisation of ore constitu ents from a deep-seated metasedimentary reservoir.